The invention relates to a method for the compensation of steering wheel rotary oscillations during operation of a steering system in a vehicle, wherein the steering system comprises an activatable torque actuator.
The invention also relates o a device for the compensation of steering wheel rotary oscillations in a steering system, wherein the steering system comprises an activatable torque actuator.
Steering wheel rotary oscillations occur in a vehicle particularly when there are imbalances on the wheels. Steering wheel rotary oscillations are also referred to as “shimmy” or “smooth road shake”. Such steering wheel rotary oscillations are usually damped or reduced by various measures since they are perceived as disruptive. By way of example, it is known to partially compensate imbalance-induced oscillations by means of design measures, such as for instance, the use of damping elements. It is also known to provide additional electronic control elements and/or sensors and to reduce the steering wheel rotary oscillations by means of appropriate activation of the control elements.
Electric power steering systems (EPS) have a torque actuator known as an EPS motor. To reduce steering wheel rotary oscillations, it is known to provide suitable activation of the EPS motor. By way of example, the current torsion bar torque is detected by means of a torque sensor. High-frequency components of the steering torque are defined as a disturbance and are damped via a characteristic that is dependent on the vehicle speed and the steering speed as well as the torque. In this case, however, among other things, desired feedback from the road is incorrectly detected as a disturbance and is therefore eliminated or at least reduced. This has negative effects on the steering feel and can bring disadvantages in terms of safety since important information regarding the road may no longer be available to the driver. Such a system is known, for example, from EP 1 839 998 A1.
EP 1 650 106 B1 also discloses a method for reducing steering wheel rotary oscillations, in which counter-oscillations are introduced into the steering system at an appropriate point by means of an electric motor. As a result, rotary oscillations that occur are partially compensated. In the known method, a current steering wheel torque is required, which is examined for the presence of periodic oscillations within a time window of suitable size. This method requires an additional torque sensor s well as time-consuming calculation of the compensation torque by means of an autocorrelation or cross-correlation function.
A method for suppressing a steering wheel rotary oscillation is known from US 2009/0000857 A1. In the known method, the angular velocity a rotor of an electric motor that assists the steering is determined. Using this variable, a compensation current is calculated by means of a frequency estimation unit, a phase correction unit and a vibration suppression unit which is positioned thereon and which requires the calculated frequency and phase as input variables. Due to the servo connection to the steering, however, the angular velocity of the rotor is only somewhat suitable as a measurement variable. Particularly in the event of a low power level on the rack, the rotor angular velocity cannot readily be used as a sufficiently accurate measurement variable.
A method for detecting steering wheel rotary oscillations based on a FFT (Fast Fourier Transform) of a torsion bar torque is known from JP 2004-161073. When a steering wheel rotary oscillation is detected in this known method, the system damping as a whole is increased. However, this has the disadvantage that the steering feel as a whole is impaired. An improvement to this method is known from EP 1 975 040 81, in which the FFT is determined more efficiently due to the fact that it is calculated only in narrow frequency bands, as a function of a current vehicle speed.
US 2009/0125186 A1 discloses another method for the compensation of steering wheel rotary oscillations, in which a filtered torsion bar torque is determined and a compensation variable which corresponds to the filtered torsion bar torque but which is phase-shifted through 180° is switched on for compensation purposes.
The problem addressed by the invention is that of providing a method and a device which make it possible to compensate steering wheel rotary oscillations as fully as possible without having any negative effects on the steering feel.